The present invention relates to devices for measuring the position of receiving antennas relative to transmitting antennas using multiple frequency magnetic fields. Particularly, though not exclusively, the disclosed devices measure position in six degrees of freedom, namely motion of translation in three coordinate directions (location) and rotational motion about three coordinate axes (orientation), location being commonly defined by X, Y, and Z linear coordinates referring to three mutually perpendicular directions and orientation being commonly described by pitch, roll, and azimuth angular coordinates about three mutually perpendicular axes usually coincident with the three mutually perpendicular directions.
As used herein "position" means location and/or orientation location.
The concept of using transmitting and receiving components with electromagnetic coupling is well known in the prior art with respect to biomechanics and computerized animation, where a subject or performer wears a number or receiver components at known strategic locations on the body, typically major arm and leg bones, shoulders, spine, hands, feet, and head. This information is then used by computing systems to precisely show the relative motions of the points in question, giving a computer generated image realistic movements. When conductive materials are present, they generate eddy current fields, which distort the received magnetic field waveform, which distorts the output of the system unless the system utilizes some distortion reducing technique.
U.S. Pat. No. 4,849,692 (Blood), U.S. Pat. No. 4,945,305 (Blood), and U.S. Pat. No. 5,640,170 (Anderson) describe the use of time domain techniques to wait for the eddy current fields to decay fully. U.S. Pat. No. 5,767,669 (Hansen and Ashe) describes a system which uses time domain techniques to measure the eddy field distortion component directly and removes it from the received signal. The disadvantage of these systems is that the receivers are quite sensitive to the low frequency (under 30 Hz) error components induced in their outputs when the sense coils are rotated in the earth's magnetic field. This motion induced component can be much larger than the component of interest when transmitter to receiver distance is large, as the transmit field falls off with the cube of distance while the Earth's field remains essentially constant. Also, since these systems use sequential time division multiplexed transmitter signals, each transmitter must be energized and sufficient time must be given for the eddy components to decay or stabilize before a receiver measurement is taken. In practice this time is on the order of 2.5 mS. Since there are generally 4 distinct transmitter states consisting of sequential energization of the X, Y, and Z coils, plus one "off" interval, these systems require approximately 10 mS to acquire a sample while maintaining reasonable eddy current distortion rejection. This translates to an effective receiver sample rate Fs of 100 hertz. Since the filters on the receivers must be designed to respond to the rapidly changing transmitter signal and must not contribute to output distortion, they are typically set at a bandwidth Fp of 2 KHz. It is thus seen that the aforementioned systems inherently do not meet the Nyquist criteria, which state that a sampled system must not have frequency components present at the sampling input which exceed one half of the sampling rate if aliasing is to be avoided. External interference signals and noise between Fs and 2 Fp are thus aliased. Of particular concern are the harmonics of the power grid, which extend from 50 Hz to beyond 1 KHz. These frequency components are aliased into the passband of the system digital lowpass filters. The aliased components are quite noticeable at bandwidths of greater than 1 Hz, beyond which the output data can and does become excessively noisy in many common environments. The narrow passband required to avoid this phenomenon results in the system having a very slow response, which is unacceptable for many applications. The noise is also increased, as the ratio of Fs/2 to Fp is typically 40:1, meaning the system samples at 100 Hz while the low pass filters pass all frequency components up to 2 KHz. A system satisfying the Nyquist criteria would need to sample at 4 KHz. Since noise is proportional to the square root of bandwidth, it can be seen that, given an equivalent set of operating parameters, the time domain eddy current compensation systems will be several times noisier than a system satisfying the Nyquist sampling criteria.
The following prior art is also known to Applicant:
U.S. Pat. No. 5,168,222 (Volsin and Monin) discloses a sequentially energized transmitter producing AC magnetic fields. Such a system will not satisfy the Nyquist criteria since the individual transmitter coils must be energized for at least 2 pi/.omega. seconds as dictated by claim 1(e). Such timing results in aliasing of the received signals. This system uses an in phase (I) and quadrature (q) detection scheme at a single frequency to reduce metal distortion effects. Of special interest is the q term, whose value is influenced both by the proximity of the metal and its conductivity. A metal plate of given area, thickness, and spatial position relative to the system in question will result in a particular value of the q term which is then used to correct the error in the I term due to the eddy field of the metal. By changing the thickness or composition of this plate (e.g. from aluminum to stainless steel), a different value of position will be obtained out of this system without moving either the transmitter or receiver, since the system does not attempt to correct for variations in the q term due to conductivity variations in nearby metallic structures.
U.S. Pat. No. 5,347,289 (Elhardt) discloses a system in which the detection method is independent of the amplitude of the received signal, this characteristic being achieved by using rotating magnetic field vectors. The system computes position based on timing information derived from the sensing of the rotating field vector, formed by the summation of fields at a point in space due to the sin(.omega.t) transmit axis and the cos(.omega.t) transmit axis. Mathematically, this equation is A cos(.omega.t)-B sin(.omega.t)=sqrt(A 2+B 2)*cos(.omega.t+(tan-1 (B/A)). Since the system is concerned with the arrival of the vector at the receiver in some steady state condition, it must then also be sensitive to the steady state phase shift term given as the inverse tangent of B/A. Due to the fact that the sin(.omega.t) and cos(.omega.t) fields are not spatially co-located in both position and orientation, it is obvious that each will pass through a somewhat different region of space. If there is metal in the environment, then the received field will be distorted in amplitude by the metallic eddy currents. Since the two field components do not pass through the same space, it follows that they can and in fact do incur different amounts of eddy current distortion. This has the effect of changing the B/A ratio, and it follows that the phase term tan-1(A/B) is thus distorted. Since the timing information in the system is derived from the phase of the received field, and the phase of the received field is susceptible to distortion by metallic eddy currents, it follows that the system position output will be degraded by the presence of metal. The device described also does not correct for variations in the system response due to conductivity changes in the metal environment. Thus, changing the composition, thickness, or shape of metallic objects near the system will cause the system to output different values for position even when the receiver and transmitter positions do not change. In column 10 and FIG. 7, a system utilizing 2 frequencies is disclosed. One frequency is 11 KHz and the other 30 to 100 Hz. The low frequency component is used to provide an accurate but slow update, while the higher frequency is used to provide a less accurate but higher update rate signal. Through some undescribed methodology it appears that the goal is to combine the two outputs into one single accurate but rapid output. This system is, thus, essentially, the combination of two prior art systems using an appropriate filter to combine the two outputs into one. Such a system inherently results in nonlinearities of the output both at the beginning and end of receiver motion, as the system utilizes the more rapid but less accurate sensing means during the period of motion, but, when motion stops, must somehow transition to a different position derived from the slower but more accurate sensing means. This generally results in a period of undesirable false motion, where the position output continues to change from the less accurate value to the more accurate value during a period where the receiver is stationary.
U.S. Pat. No. 5,640,170 (Anderson) discloses a transmitter configuration using a spiral conductor pattern over a conductive ground plane in order to produce a low distortion field over the configuration. This method of reducing conductive material eddy current distortion is not related to the method disclosed in this patent.